What is spiritual development? In Christian terms, it is a soul becoming more Christlike: fully in the image of God, but humble and fully human. Christ's life continuously showed to humanity the signs of both the transcendence and the humble love of God. He lived a life where his words and actions were full of beauty of every type, from compassion to judgment to mercy to love.
Christ is the template that transforms our lives. To develop spiritually is to become more Christlike. The beauty of Christ's life reshapes our own.